More than 35,000 Iraqis have been detained by American troops since the invasion of the country but only a tiny fraction have been convicted of wrongdoing, the Guardian has learned.

About 21,000 have been released without ever being charged or tried. Of the 1,300 who have been charged, only half have been found guilty.

Some 13,500 Iraqis are still being detained, more than double last year's total, according to official American figures. The figures were obtained by the Liberal Democrats from sources in US Central Command, which oversees all American military operations in Iraq.

Most of the "security detainees" still being held are in "theatre internment facilities" including Abu Ghraib, the prison at the centre of the scandals of abuse by US troops. Others are being held in division or brigade internment centres.

The US military says that "security detainees" are held under UN security council resolution 1546 and article 78 of the fourth Geneva convention covering the protection of civilians in times of war. This raises the question of whether the US military is still fighting a war, and whether it believes it is.

The US argues that the detainees do not have the right to legal advice. However, the only reference to detention under the UN resolution is an appended letter by Colin Powell, then US secretary of state, the Lib Dems say. The Powell letter says multinational forces in Iraq would impose "internment where this is necessary for imperative reasons of security".

A detainee can be charged only when their case has been investigated by an Iraqi judge. Only then does the detainee have access to legal advice.

Human Rights Watch, the New York-based campaigning and research organisation, early this year referred to "the systematic use of arbitrary arrest, prolonged pretrial detention without judicial review, torture and ill-treatment of detainees, denial of access by families and lawyers to detainees, improper treatment of detained children and abysmal conditions in pretrial detention facilities".

In a second report, in April, it said there was overwhelming evidence that US mistreatment and torture of Muslim prisoners took place not only at Abu Ghraib but at facilities throughout Afghanistan and Iraq. It concluded that "a good number of the victims were civilians with no connection to al-Qaida or terrorism".

The Lib Dems argued yesterday that there was reason to believe that a significant proportion of those who had been detained were joining, or rejoining, the insurgency after their release. The US system of detentions may actually be fuelling the insurgency, they argue.

Official US figures help support this. The number of Iraqis detained by the US and other foreign forces has more than doubled in a year and a half. The number of attacks has also more than doubled. "It is difficult to think of anything better calculated to create antagonism among the Iraqi population than detention against which there is no right to challenge or to appeal," the Lib Dem foreign affairs spokesman, Sir Menzies Campbell, said yesterday. He added: "Acting wholly contrary to accepted principle and without regard to legal obligations will inevitably make the struggle much more difficult. For the Iraqi government to have such a subordinate role until the point of conviction simply underlines the fact that they are a long way from having sovereignty over their own country."

In numbers

35,000 have been detained since the invasion began in March 2003

13,500 remain in detention, double last year's total

1,300 have been charged so far; only half of these have been found guilty